Sealing jaw constructed from two ceramic materials of which one is electrically conductive

ABSTRACT

The invention relates to a sealing jaw for sealing thermoplastic materials or paper or cardboard coated with thermoplastic material, preferably packaging material. The sealing jaw is constructed from two ceramic materials, of which one is electrically insulating and the other electrically conductive: The electrically conductive ceramic material forms a continuous strand embedded in the insulating material and is, at or in the proximity of its end points, metallized so as to make good connection with an electric current source. The electrically conductive ceramic materials are treated with high pressure and heat so that they together form a pore-free, hard and abrasion-resistant body which can be applied to a steel rail.

TECHNICAL FIELD

The present invention relates to an apparatus for sealing and/or joiningtogether thermoplastic materials or materials coated with thermoplastic,preferably packaging materials, by heating and compression of thematerials within the sealing zone, with a view to achieving a surfacefusion of mutually applied thermoplastic layers for the purpose ofobtaining a tight and mechanically durable seal.

BACKGROUND ART

It has long been known within packaging technology to produce packagesof thermoplastic materials or materials coated with thermoplasticlayers, for example plastic-coated paper or cardboard. The advantageinherent in these materials is that they can be joined together andsealed to one another in a convenient manner by surface fusion ofmutually applied thermoplastic layers. By such means, not only is itpossible to achieve the result that the packages, which have been formedby folding or other methods, can be maintained in their given shape, butit is also possible to achieve the result that the packages may be givena tight seal which protects the contents in the package and prevents itfrom leaking out. Seals of this type are most generally realized withthe aid of so-called hot jaws which consist of devices which canaccommodate between them the materials intended for sealing and compressthem under the simultaneous supply of heat so that the desired parts ofthe compressed and mutually facing plastic layers are caused to fusetogether into a tight and mechanically durable seam or joint. Thecommonly occurring type of such sealing jaws consists of metallic bandsor strips disposed on support rails, or metal bodies which are eitherpermanently heated or are heated on each sealing occasion by the passageof an electric current through them. Sealing may either be carried outwith the aid of two heated sealing jaws which are urged towards oneanother on either side of the material which is to be sealed, or also byone heating jaw and a so-called counter jaw whose sole purpose is toconstitute a back-up abutment for the heating jaw and cool the materialin order that the sealing seam or joint may rapidly be stabilized. Themost commonly occurring sealing jaws in modern, automatic packagingmachines consist of heat jaws with thin metal bands or strips disposedover an insulating ceramic material provided on a steel rail. At themoment of sealing, the sealing jaws are closed with great compressiveforce so that between them they accommodate the material which is to besealed, whereafter a brief current pulse is passed through theabove-mentioned metallic strips which are instantaneously heated to hightemperature. The heat generated in the metallic strips is transferred tothe packaging material and the mutually facing thermoplastic layers arecaused to fuse together. The current pulse is discontinued whensufficient quantity of heat has been generated, whereafter the sealingjaws are opened and the sealed material is released.

For production engineering and other reasons, for example those forceswhich act on the strips because of their elongation on heating, it isdifficult, employing thin metal strips, to realize anything other thanidentical and straight seals. In certain cases, there is however a needto be able to work with curved sealing lines or seals of differentstrengths or adhesive force along the extent of the seal. It has not inprinciple been possible to realize this with the aid of prior arttechnology which utilizes metallic strips but, according to the presentinvention, these needs may be satisfied.

SUMMARY OF THE INVENTION

The present invention is characterized by two sealing jaws which aredisposed to be movable in relation to one another, and are arranged tobe capable of accommodating between them the material intended forsealing, at least the one sealing jaw displaying a ceramic part whichcomprises at least two ceramic materials, of which the one iselectrically insulating and the other electrically conductive, and thatthe electrically conductive ceramic material forms one or morecontinuous strands embedded in the insulating material which are, at orin the proximity of their ends, connectible to an electric currentsource.

Thus, in the present invention, metal strips have been replaced byelectrically conductive ceramic material which, among other things,enjoys the advantage that there will be considerably smaller differencesin the coefficient of linear expansion between the conductive materialand the subjacent insulating ceramic layer. This entails that themechanical stresses in the ceramic insulating layer may be reduced.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING

One preferred embodiment of the present invention will now be describedin greater detail hereinbelow, with particular reference to theaccompanying schematic Drawing, in which:

FIG. 1 is a perspective view of a heat sealing jaw;

FIG. 2 shows a cross section taken along the line A--A in FIG. 1;

FIG. 3 shows a detail of the ceramic insulating material; and

FIG. 4 shows how the sealing cycle proceeds.

DESCRIPTION OF PREFERRED EMBODIMENT

The heating jaw illustrated in FIG. 1 is constructed on a rail 1 of, forexample, steel or copper, to which the insulating ceramic layer 2 isapplied with the aid of an adhesive 6 or with the aid of mechanicalanchorage members. As an "inlay" in the insulating ceramic layer 2,there is a strand of an electrically conductive ceramic material 3. Theelectrically insulating material 2 may, for example, consist of amixture of zirconium dioxide (Z_(r) O₂) and silicon carbide (SIC) andthe electrically conductive ceramic material 3 of a mixture of titaniumboride (T_(i) B₂) and silicon carbide (SiC). The proportion of Z_(r) O₂in the insulating ceramic may be of the order of magnitude of between 10and 50 percent by volume, preferably 30 percent by volume, and, in theelectrically conductive ceramic, the proportion of T_(i) B₂ may be ofthe order of magnitude of between 20 and 60 percent by volume,preferably 45 percent by volume. The electrically conductive material 3constitutes a continuous strand which, in this case, has been givencomplicated and varied configuration with the intention of facilitatingthe description and an understanding of the present invention. The endpoints of the strand 3 display metallized contact spots 4 to whichelectric leads 5 from a current source may be connected. As is apparentfrom FIG. 1, the extent of the strand 3 is divided into three zones I,II and III for facilitating an understanding of the followingdescription.

When heat is to be generated in the sealing heat jaw according to FIG.1, the leads 5 are connected to a current source and a current pulse ispassed through the electrically conductive ceramic material 3. Sincethis is a matter of a single current path, the electric current throughthe electrically conductive ceramic material 3 will be uniform along allparts of the current path and, in other words, will be uniform withinthe above-mentioned zones I, II and III along the strand of ceramicelectrically conductive material 3. Since the cross sectional area ofthe strand of electrically conductive material 3 is not uniformthroughout the entire current path, the current density will vary andthereby also the heat generation.

Within zone I, the current path in the electrically conductive material3 is, granted, irregular inasmuch as it is not straight, but theelectrically conductive material 3 has the same cross sectional areawithin the whole of zone I, which implies that heat generation along thecurrent path within zone I is the same per unit of length within theentire zone. On the other hand, within zone II, the strand ofelectrically conductive material 3 is of considerably greater width and,since it may be assumed that the thickness of the layer 3 has not beenchanged, the cross sectional area of the electrically conductivematerial 3 within zone II is considerably greater than within zone I. Asis well known, the electric resistance of a material is directlyproportional to the cross sectional area of the material, which impliesthat the electric resistance per unit of length in the electricallyconductive material within zone II is less than the correspondingelectric resistance within zone I. Since the heat generation which isobtained when a current is passed through a resistance is R×1², where Ris the resistance and I is the current strength, it will readily beperceived that the heat generation per unit of length in zone II will beless, since R (i.e. the resistance) is less than in zone I. Apart fromthe fact that the generated heat is less in zone II, the contact surfaceof the electrically conductive material 3 in the strand within zone IIis considerably greater per unit of length than within zone I, whichimplies that that heat which is generated within zone II is, on the onehand, less per unit of length and, on the other hand, is dissipated overa greater surface. This entails that a surface expansion of the typewhich is shown within zone II may be employed in the sealing jaw withinareas where no or slight sealing force and heat generation are desired.In zone I, sufficient heat is generated to achieve a surface fusion ofsealed material along the configuration which has been imparted to theelectrically conductive material 3, since sufficient quantity of heatwithin a relatively narrow zone has been generated. As has beenmentioned previously, not as much heat is generated within zone II, andthat heat which is generated is, moreover, dissipated over a widersurface area, which entails poorer sealing. Correspondingly, it ispossible, by local reduction of the area of the current path, to obtaina local heat concentration in those cases where such is desirable.

Within zone III, it is indicated how it is not only possible to vary thewidth of the sealing pattern and the appearance of the current path 3,but also within the portion 14 in zone III, the current path is raised afew tenths of a millimeter. This implies that, within this region, therewill be achieved a locally higher mechanical force when the sealing jawsare urged against the material which is to be sealed, and this alsocontributes in obtaining a more powerful seal. The portion 15 of thecurrent path has been shaped into an edge with whose aid it is, forinstance, possible to carry out direct cutting operations, perforations,or desired mechanical processing of the packaging material (for examplerealizing incisions, holes, slots, etc.), at the same time as thesealing operation is carried out.

It is thus possible, with the sealing jaw illustrated in FIG. 1, notonly to realize sealing lines of optional extent, but also to realizedisruptions in the sealing line, local strengthening of the seal andeven to realize slots, perforations and the execution of othermechanical processing at the same time as the sealing operation is putinto effect. By raising, on either side of the conductive material, theelectrically insulating ceramic to a level flush with the conductivematerial, cold pressure zones can also be obtained in the proximity ofthe sealing region which can thereby prevent the lateral expansion ofthe seal. Because of the extremely high abrasion resistance and hardnessof the ceramic materials, the shape which has been imparted to thematerial on manufacture is not blunted or altered.

FIG. 2 shows a cross section of FIG. 1 taken along the line A--A. Aswill be apparent from FIG. 2, the steel rail 1 is provided with drilledcooling ducts 7 or with open channels 8 which connect directly to thesurface of the ceramic insulating material. As is apparent from FIG. 2,the ceramic insulating material 2 is directly connected, by means of anadhesive layer 6, to the steel rail I and the electrically conductivematerial 3 is inlaid into the insulating ceramic material 2. Since theceramic materials 2 and 3 are substantially of the same coefficient oflinear expansion, no appreciable stresses will occur between the ceramicelectrically insulating layer 2 and the electrically conductive materiallayer 3. Granted, the conductive layer 3 will be heated considerablymore than the insulating layer 2, but an efficient cooling with the aidof the cooling ducts and channels 7 and 8 entails that the probleminherent in mechanical stresses in the material can be controlled.

The working method for the heat sealing jaw according to FIG. 1 isschematically illustrated in FIG. 4 in which the material 11 intendedfor sealing (and consisting of a packaging material with a paper layer12 and plastic coatings 13) has been inserted between one heat sealingjaw and one so-called counter jaw 10. The heat sealing jaw and thecounter jaw are urged towards one another while accommodating andcompressing the packaging material 11 between them. In connection withthis compression, a current is passed through the electricallyconductive material layer 3 which is instantaneously heated, whereafterthe heat is transferred by conduction to the plastic layers 13 of thepackaging material 11 which is caused to melt and, under the prevailingpressure, to fuse and unite with one another. In order rapidly to beable to stabilise the thus achieved seam or joint, i.e. to cause theplastic material once again to be cooled, the counter jaw 10 is, in theillustrated embodiment, provided with cooling channels 9. Aftercompleted sealing operation, the sealing jaws are separated, whereafterthe sealed or joined packaging material 11 can be removed.

By way of introduction, the manner of manufacture of a heat sealing jawin accordance with the present invention was touched upon lightly andwill be explained in greater detail below.

The manner of manufacture is based on the concept that the ceramicmaterial may be produced in two steps. In a first step, a body of theinsulating material 2 is produced by pressing of a ceramic powder, andone such body is schematically illustrated in FIG. 3. A channel orincision 16 corresponding to the form of the desired pattern of extentof the electrically conductive ceramic 3 is also impressed or shaped inthe body 2. The cavity or channel 16 is thereafter filled with ceramic,electrically conductive powder material, the strand 3 being thus formed.

That material of which the body 2 consists comprises a powder which ispressed to a configuration suitable for the purpose, possibly with someform of retroprocessing. The material in the strand 3 may consist of apowder which is pressed into the cavity 16, or alternatively mixed intoa slurry which is coated over the body 2.

This pressed ceramic body does not, however, possess any mechanicalstrength of note, for which reason it must be handled with a certaindegree of care. The formed ceramic body or, more precisely, thecombination of different ceramic materials with different electricproperties, is placed in a pressure chamber. In this pressure chamber(entitled the Quintus Press by the manufacturers ABB), the pressure israised to approx. 160 MPa and the temperature to 1750° C. After theceramic materials have been subjected to heat and high pressure forapprox. 4 hours, they have sintered together to form an extremelyfine-pored, hard and abrasion resistant body which, in a mannermentioned above such as, for example, by soldering or gluing, is securedto the cooling body or alternatively is mechanically clamped in place.

It has proved that, using the new technology employing ceramicelectrically conductive material, considerable freedom is afforded inselecting the appearance, extent, local strength, etc., of seals and ithas moreover proved that the ceramic heat sealing jaws possess excellentabrasion resistance, for which reason they are not worn out to anyappreciable degree. It is naturally possible, without departing from thespirit and scope of the inventive concept as herein disclosed, torealize completely different sealing configurations and practicalapplications than those presented here, and it is also possible toemploy other ceramic materials than those disclosed, even though it hasbeen found that these particular ceramics which have been mentioned hereare the most expedient of those ceramic materials or ceramics known tous. It is often of importance to maintain extreme precision and accuracyin the design and construction of a sealing jaw, and it may, thereforebe necessary, after production of the sealing jaw according to thosemethods accounted for in the foregoing, to grind the electricallyconductive strand 3 in order, on the one hand, to obtain an improvedsurface smoothness, and, on the other hand, a greater degree ofdimensional precision. It may also prove necessary and appropriate, inthe production of the sealing jaw, first to produce and pressure treatthe electrically insulating material 2 for realizing a body inaccordance with that illustrated in FIG. 3. Thereafter, the channels 16in the body 2 may be filled with electrically conductive ceramicmaterial 3, whereupon the entire body is once again subjected topressure and heat treatment so that the ceramic material 3 also sinterstogether to form a hard and abrasion resistant part. A simultaneouspressure treatment and sintering of the two ceramics is, however, to bepreferred and this gives the best union strength between theelectrically conductive and the electrically insulating ceramics.

The present invention should not be considered as restricted to thatdescribed above and shown on the Drawing, many modifications beingconceivable without departing from the spirit and scope of the appendedclaims.

What is claimed is:
 1. A method for making a sealing jaw adapted to sealthermoplastic materials, said sealing jaw having an electricallyinsulating ceramic layer a heating element, which is imbedded in saidinsulating layer, first connecting means, which is located in proximityto one end of said heating element, and second connecting means, whichis located in proximity to an opposite end of said heating element, saidfirst and second connecting means connecting said heating element to asource of electric current, and said heating element being made from anelectrically conductive ceramic material and cooperating with said firstand second connecting means to form an electric current conductingpathway, whereby electric current flowing through said heating elementcauses the direct heating of said heating element, said methodcomprising the steps of forming ceramic powders into bodies of desiredconfiguration for said insulating layer and said heating element;uniting said bodies with one another into a combined body; heating saidcombined body to a range between 1600° C. and 1900° C. under a pressureexceeding 100 MPa for at least 120 minutes, whereby the ceramic powdersare sintered together to form an integral, dense and pore-free structurewhich is extremely hard and abrasion-resistant.
 2. The method of claim1, further comprising the steps of applying said bodies in layers aboveone another and processing said structure by grinding same to desireddimensions.
 3. Apparatus adapted for use in producing packages which areformed from a tube produced from a planar web of packaging material, thetube being, after filling with the intended contents, divided up intopackaging containers by repeated sealings transversely of thelongitudinal direction of the tube, said apparatus comprising a sealingjaw having an electrically insulating ceramic layer a heating element,which is imbedded in said insulating layer such that at least a portionof said heating element is inlaid in said insulating layer, said atleast a portion of said heating element and said insulating layer beingsintered together so as to form an integral, fine-pored, hard andabrasion-resistant body; first connecting means, which is located inproximity to one end of said heating element; and second connectingmeans, which is located in proximity to an opposite end of said heatingelement, said first and second connecting means connecting said heatingelement to a source of electric current, and said heating element beingmade from an electrically conductive ceramic material and cooperatingwith said first and second connecting means to form an electric currentconducting pathway, whereby electric current flowing through saidheating element causes the direct heating of said heating element.
 4. Asealing jaw for sealing thermoplastic materials, comprising anelectrically insulating ceramic layer; a heating element imbedded insaid insulating layer such that at least a portion of said heatingelement is inlaid in said insulating layer, said at least a portion ofsaid heating element and said insulating layer being sintered togetherso as to form an integral, fine-pored, hard and abrasion-resistant body;first connecting means located in proximity to one end of said heatingelement; and second connecting means located in proximity to an oppositeend of said heating element, said first and second connecting meansconnecting said heating element to a source of electric current, andsaid heating element being made from an electrically conductive ceramicmaterial and cooperating with said first and second connecting means toform an electric current conducting pathway, whereby electric currentflowing through said heating element causes the direct heating of saidheating element.
 5. The sealing jaw of claim 4, wherein said heatingelement includes another portion extending above a surface of saidinsulating layer.
 6. The sealing jaw of claim 5, wherein said insulatinglayer includes a channel formed in said surface thereof and wherein saidat least a portion of said heating element has a shape complementary tothe shape of said channel.
 7. The sealing jaw of claim 6, furthercomprising a metal rail, said insulating layer being fixedly attached tosaid rail.
 8. The sealing jaw of claim 6, wherein said rail includes aplurality of cooling ducts formed therein.
 9. The sealing jaw of claim6, wherein said surface of said insulating layer includes elevated areaswhich are flush with said heating element for providing cold pressurezones, which function to inhibit lateral expansion of a seal inthermoplastic materials during the performance of a sealing operation bysaid sealing jaw.
 10. The sealing jaw of claim 6, wherein thecoefficient of linear expansion of said insulating layer issubstantially similar to the coefficient of linear expansion of saidheating element.
 11. The sealing jaw of claim 4, wherein said heatingelement includes first and second portions having first and secondheights, respectively, said first height being greater than said secondheight, whereby said first portion exerts greater mechanical sealingforces upon thermoplastic materials than said second portion during theperformance of a sealing operation by said sealing jaw.
 12. The sealingjaw of claim 11, wherein said first portion of said heating element hasan edge at a surface thereof opposite said insulating layer forperforming a mechanical operation during the performance of a sealingoperation by said sealing jaw.
 13. The sealing jaw of claim 12, whereinsaid mechanical operation includes a cutting operation.
 14. The sealingjaw of claim 12, wherein said mechanical operation includes an operationfor forming at least one perforation in thermoplastic materials.
 15. Thesealing jaw of claim 12, wherein said mechanical operation includes anoperation for forming at least one incision in thermoplastic materials.16. The sealing jaw of claim 4, wherein said heating element includes aportion extending longitudinally along said insulating layer in anon-linear fashion for providing a non-linear sealing line inthermoplastic materials.
 17. The sealing jaw of claim 4, wherein saidheating element includes first and second portions having first andsecond cross-sectional areas, respectively, transverse to thelongitudinal axis of said heating element, said first cross-sectionalarea being larger than said second cross-sectional area, whereby saidfirst portion generates less heat than said second portion when electriccurrent flows through said heating element.
 18. The sealing jaw of claim4, wherein said insulating layer is formed from a mixture of zirconiumdioxide (ZrO₂) and silicon carbide (SiC), the quantity of zirconiumdioxide constituting between 10 and 50 percent by volume, and whereinsaid heating element is formed from a mixture of titanium boride (TiB₂)and silicon carbide (SiC), the quantity of titanium boride constitutingbetween 20 and 60 percent by volume.